REUTERS - The Floyd Mayweather and Conor McGregor crossover fight on Saturday has been denounced by boxing purists as a farce and a circus but is not be the first time athletes from different worlds have clashed to decide who is the best.

Tennis, athletics and motor racing have crossed gender lines, age barriers and two and four wheels in an elusive quest for greatness. But in the end the results have seldom produced much beyond criticism and laughs.

Few historical comparisons spring to mind when it comes to the fight between undefeated boxer Mayweather and mixed martial arts champion McGregor but Muhammad Ali’s farcical bout against Japanese professional wrestler Antonio Inoki in 1976 is perhaps the nearest one can find.

The circus-like feel in Tokyo made it feel more like a Ringling Brothers production than a serious sporting contest.

“That was a farce and this is a farce,” Top Rank Boxing chief executive Bob Arum, who was Ali's promoter for many of his fights, said in a telephone interview in which he likened Saturday's fight to the movie ‘Dinner for Schmucks.'

“This is ‘Event for Schmucks.’ At least Ali versus Inoki one was supposed to box and one to wrestle.

“McGregor is okay as an MMA fighter but he's a boxing neophyte.”

As with Saturdays' fight in Las Vegas, the Ali-Inoki contest pitted two champions from different disciplines going head-to-head.

The rules for Ali v Inoki were negotiated between the two camps and were a hybrid of boxing and wrestling. Inoki spent nearly the whole fight lying on the canvas kicking at Ali’s legs, and inflicting serious damage, including two blood clots.

The bout, declared a draw, was panned by the viewing public and critics alike, but it played a role in the evolution of mixed martial arts by showing that boxing and wrestling could be combined into one sport, albeit with better defined rules than the Tokyo affair.

Arum said the length of Saturday's fight would be in Mayweather’s hands -- literally.

"It will last as long as they want it to last, two or three rounds maybe. He (Mayweather) can knock McGregor out whenever he wants.

"He can take his time. Hopefully he can maintain himself until the second or third round.

"It’s a typical con. That being said, nobody’s holding a gun to anyone’s head. I have no sympathy for anyone who buys (the TV pay-per-view) and then complains.”

Farce or no farce, at least Mayweather and McGregor will do very well financially from the fight.

That’s a far cry from the sad sight of Olympic sprint great Jessie Owens racing against horses back in the 1940s in an effort to earn a few bucks and stave off poverty.

A 150 metre showdown between Canadian 100 metre Atlanta Olympic champion and world record holder Donovan Bailey and American 200m gold medalist and record holder Michael Johnson of the United States also decided nothing and ended in jeers when the American pulled up lame mid-race.

Another crossover sporting contest was the 1973 tennis battle of the sexes between Billie Jean King, at the time one of the best female players in the world, against former Wimbledon champion Bobby Riggs, then aged 55.

But that match proved anything but farcical, as King won in straight sets, considered a milestone in the women’s liberation movement, showing that sometimes it is difficult to predict how an event will shape sporting history.

Some like John Surtees have moved easily between sports the Briton the only man to win world titles on two wheels and four, capturing the 1964 Formula One world drivers tile and four 500cc and three 350cc motorcycling world titles.

(Reporting by Andrew Both in Cary, North Carolina; Editing by Steve Keating)